1. Field of the Invention
This invention relates to pressure sensitive conductors (pressure sensitive conductive rubbers) which have high mechanical strength, rapid response to electrical deformations, low hysteresis of the electrical resistance variation on application and release of pressure and very limited variation of the pressure-resistivity curve under repeated deformations.
The term "pressure sensitive conductors" is used in this application to refer to the rubber-like resistors or conductors whose volume resistivity (.OMEGA..multidot.cm) varies in the order of three digits or more when pressure is applied thereto.
2. Description of the Prior Art
It is known that composite products obtained by mixing conductive metal particles in an elastomer and then molding and cross-linking the mixture can provide a pressure sensitive conductor whose volume resistivity is greatly varied when the conductor is deformed.
Nevertheless, there are very few pressure sensitive conductors utilized on an industrial scale. The unfavorable reception of such conductors by industry has been attributed to their unfortunate tendency to change their resistance under no load constitions, their unsatisfactory overall electrical properties and their poor mechanical strength or durability.
Most pressure sensitive conductors containing metal can stand only approximately 100,000 repetitions of use. Undesirable hysteresis phenomena and changes in electrical properties take place as the number of times of use increases. As a result pressure sensitive conductors are still unsatisfactory in respect of reliability and continuous efficiency.
It is known that conductive rubber can be obtained by blending conductive carbon black with a solid rubber such as silicone rubber or the like and then molding and cross-linking the blend. This conductive rubber is widely utilized for switch elements and in other applications. However, the mixture of rubber and conductive carbon black can not function as a pressure sensitive conductor which is substantially an insulator when no pressure is applied and becomes conductive only with the application of pressure. It is known that pressure sensitive conductors obtained from such carbon black-rubber blends alone show a volume resistivity (.OMEGA..multidot.cm) change in the order of only about two digits on application of pressure.
Dutch Patent Application No. 65-14033 discloses an electrical conductor produced by dispersing carbon black and conductive metal particles in a rubber, but this product is not a pressure sensitive conductor of this invention because the quantity of the conductive metal particles is too low and the carbon black is too high.